The present invention relates to an optical amplifier apparatus for use in amplifying low power, reduced frequency, ultrashort pulse seeds and in particular, though not exclusively, to an optical amplifier apparatus having an integrated pre-amplifier and power amplifier in a single crystalline slab active region.
Lasers with pulse widths of less than 10 ps offer new processing capabilities in micro-machining industrial applications. However, the processing speeds enabling mass manufacture require pulse repetition frequencies of between 100 kHz to 10 MHz, adjustable to an optimal frequency with average powers in excess of 100 W. Typically, such combinations are not achievable with most laser architectures as the maximum pulse energy is limited by non-linear effects and damage to the laser crystal. Thin disk laser oscillators have achieved powers approaching 150 W, being at the lower end of the power range required, but at fixed pulse repetition frequencies of between 3.50 MHz and 60 MHz, being above the required range, and they cannot easily be adjusted to an optimal frequency for a specific process whilst maintaining the average power.
Currently, master oscillator power amplifiers (MOPA's) are used to obtain the high average powers required. In such apparatus a low power laser master oscillator generates pulses of the required width which are coupled into a power amplifier. The input pulse stimulates emission within the amplifier which is added to the input pulse to create a higher output energy pulse. As both the intensity and fluence are significantly lower than would be achieved within an oscillator having a similar output, the apparatus can achieve higher output power and energies before damage occurs. MOPA's are now being implemented in a number of alternative laser architectures.
U.S. Pat. No. 6,654,163 to Fraunhofer-Gesellschaft zur Foerderung der angewanten describes an optical amplifier apparatus which is found in the commercially available INNOSLAB amplifier from, for example, EdgeWave GmbH, Germany. As illustrated in FIG. 1, this apparatus has a rectangular crystalline slab amplifier medium A. Beam B, which is emitted by an oscillator, travels a path C between mirrors D,E in which the beam B traverses the amplifier medium A multiple times. With each traverse of the medium A, the cross section of the beam B increases in the x-direction. The beam size is maintained in the thin y-direction. The expansion along the x-axis is chosen to ensure the beam intensity is held nearly constant as the beam is amplified. The number of traverses is chosen to maximise overlap between the beam B and amplifier medium A. In this way, using a single pass through the amplifier, stored energy can be efficiently extracted whilst the thresholds for damage and non-linear effects are avoided. Using such apparatus, average power levels of 400 W with a pulse width of 680 fs at a frequency of 76 MHz have been achieved.
A disadvantage of this arrangement is that sufficient input power is required to ensure effective saturation occurs in the initial few passes through the amplifier.
Amplifier systems which then cascade such oscillator-amplifier apparatus with a second, rectangular slab amplifier, where the beam makes a path of one traverse in a single pass, can achieve power levels of up to 1.1 kW with a pulse width of 615 fs at a frequency of 20 MHz. Again, these amplification approaches rely upon sufficient input seed power to ensure effective extraction. In addition, a cascaded arrangement requires considerable space.
To overcome the disadvantage of requiring sufficient input seed power, amplification of lower seed powers using regenerative amplifiers or pre-amplification stages have been proposed. U.S. Pat. No. 7,903,715 to Gigaphoton Inc. details use of a regenerative amplifier in an alternative laser architecture. An embodiment is shown in FIG. 2 where one amplification medium slab F is used to perform the multiple functions of an amplifier G arranged in the manner of FIG. 1, with a single pass of multiple traverses of the active medium F, and a regenerative amplifier H, with multiple passes of a single traverse of the active medium F. A low power seed laser J is injected into a resonator formed of two mirrors M, N. The injected signal is introduced by switching a photoacoustic element K. The beam B then makes multiple passes in a single traverse between the mirrors M, N until the majority of the stored energy is extracted. In a desired timing, a Pockels cell L is switched and the beam B is output from the regenerative amplifier H into the amplifier G using a polariser P. The arrangement is compact in that the amplifier G and the regenerative amplifier H share the same active medium F, but they are separated from each other in the medium F and operate as largely independent devices.
A disadvantage of this arrangement is in the requirement for active components. Such Pockels cells and photoacoustic elements add cost and complexity to the arrangement.
It is an object of the present invention to provide an optical amplifier which provides amplification for low power, reduced frequency, ultra-short seed pulses.
It is a further object of at least one embodiment of the present invention to provide an optical amplifier which integrates a pre-amplifier and a power amplifier in an active medium with partial coupling of the pre-amplifier and the power amplifier.
It is a still further object of at least one embodiment of the present invention to provide an optical amplifier which uses purely passive components.